Excessive concentrations of nitrate in water in the environment can cause health hazards and can cause accelerated eutrophication of estuarine waters. The Maximum Contaminant Level of nitrate in drinking water as established by the United States Environmental Protection Agency (EPA) is 10 milligrams per liter. Excessive concentrations of nitrate have been linked to methemoglobinemia or “blue baby” syndrome.
The treatment of wastewater easily removes almost all organic carbon from the treated fluid. Nitrogen is more difficult to remove from wastewater. The final effluent from wastewater treatment facilities usually has more nitrogen than carbon on a mass basis. The discharge of effluent with more nitrogen than carbon will create an environment in the soil regime and ground water that will allow the nitrogen to pass long distances in the ground water with minimal microbial attenuation. Nitrate is the limiting nutrient in salt water and estuarine systems. Dissolved nitrates in groundwater and surface waters draining to estuaries cause accelerated eutrophication of salt-water environments.
Nitrate is an inorganic form of nitrogen. Nitrate is the result of bacteria driven oxidation of organic nitrogen and ammonia in two steps. Initially, bacteria Nitrosonomas and other species oxidize ammonia (NH4) into Nitrite (NO2−). The bacteria Nitrobacter and other species then oxidize Nitrite into Nitrate (NO3−). These bacteria are prevalent in all soils. These bacteria are slow growers and tend to take a long time to reach effective population density to provide complete nitrification of the organic nitrogen.
The removal of dissolved nitrate from water is called denitrification. Denitrification is a microbially driven reaction, in anoxic conditions, where an available carbon is present. The carbon has to be readily available or capable of being used readily by bacteria for metabolism. In anoxic conditions in waters, time, temperature, bacteria and available carbon limit denitrification. If there are sufficient conditions for denitrification, then the dissolved nitrate will be completely denitrified with the end products being free nitrogen gas, free carbon dioxide gas, dissolved organic nitrogen and dissolved organic carbon in the water and organic nitrogen and organic carbon in biomass. This biomass occurs as slime. The slime can result in biofouling in any voids if denitrification occurs in soils, packed bed filters or in sand filters.
There are several methods to induce the denitrification of wastewater. Regardless of the method chosen in the treatment process, there are measurable concentrations of Total Nitrogen (TN) in the final effluent. The final effluent always has dissolved organic nitrogen and dissolved inorganic nitrogen.
The removal of dissolved nitrogen in water will be described as the removal of TN. The concentration of TN is the total of the concentrations of Total Kjedlhal Nitrogen (TKN) plus NO2− and NO3−. Dissolved organic nitrogen is described as TKN minus Ammonia (NH3). TKN is defined as the total of ammonia concentrations and dissolved organic nitrogen concentrations. Inorganic nitrogen is measured as the total of the concentrations of (NO2−) and (NO3−) and NH3. The value of TN then is the addition of NO2− plus NO3− plus TKN.
The removal of nitrogen has to be quantified as the removal of TN to account for all nitrogen compounds. For instance, denitrification of NO3− will result in the discharge of low concentrations of TKN as a byproduct. This TKN will by microbially oxidized in the environment back into NO3−. As a result, nitrogen removal is quantified by measuring the difference in TN.
Bacteria in soil typically have a carbon to nitrogen ratio of 3:1 to 5:1. In order to enlist the soil bacteria to address nitrogen pollution discharged effluent should have sufficient dissolved carbon for denitrification and microbial metabolism. The typical practice in the treatment of wastewater is to discharge treated effluent with very low carbon concentrations as measured by Biochemical Oxygen Demand (BOD5). Standard practice is evolving to also remove TN to low concentrations in the final effluent. These two factors then limit the potential for natural attenuation in the soils below the soil absorption systems of wastewater treatment systems and wastewater treatment plants. Plumes of nitrogen from treatment plants then tend to travel for miles due to the fact that there is insufficient carbon in situ and in the ground water to promote significant denitrification.
Biochemical Oxygen Demand (BOD5) is a standard test used to determine a measure of the organic strength of wastewater or the amount of organic carbon dissolved in water. BOD5 is the rate at which organisms use the oxygen in water or wastewater while stabilizing decomposable organic matter under aerobic conditions. The BOD5 value is calculated based on the difference between two dissolved oxygen measurements of a sample. The first measurement is taken at the time of collecting the samples and the second measurement is taken after 5 days of incubation. After five days, there is still some remaining dissolved organic carbon that will slowly degrade over time.
With the increase in population and increased development in coastal areas, there have been greater impacts of nitrogen compounds on drinking water and the water quality in estuarine waters. The discharge of treated effluent with excessive levels of nitrate can lead to a public health problem. The EPA has established a Maximum Contaminant Level of ten (10) milligrams per liter of NO3− in drinking water. Much lower concentrations of NO3− in ground water draining to estuaries can cause degradation of coastal wetlands and coastal waters.
Therefore, there is a need for a method for further reducing nitrogen pollution in treated wastewater and in the ground water affected by the discharge of treated effluent from wastewater treatment facilities into the ground.
There is a need to treat any water with nitrogen pollution and lower the concentration of TN in the water.